Question

Which elements in the list below are capable of forming molecules in which one of its atoms has an expanded octet? Explain your answer. a. \(B\) b. \(C\) c. \(P\) d. \(O\) e. Se

Short answer

The elements in the given list that can form molecules with one of their atom having an expanded octet are P and Se. This is because they are in period 3 or below and have access to d orbitals, allowing them to form expanded octets.

Step-by-step solution

Periodic Table Position

To understand the position of each given element in the periodic table, let's list their periods and groups. a. B: Period 2, Group 13 b. C: Period 2, Group 14 c. P: Period 3, Group 15 d. O: Period 2, Group 16 e. Se: Period 4, Group 16

Eliminating Elements with No Access to d Orbitals

Elements in Period 2 do not have access to d orbitals, and hence cannot form expanded octets. So we can eliminate the following elements: a. B b. C d. O

Analyzing Remaining Elements for Expanded Octet Capability

Now we have only two elements left: c. P: Period 3, Group 15 e. Se: Period 4, Group 16 Both of these elements are in period 3 or below, and hence have access to d orbitals. Thus, they can potentially form molecules with expanded octets.

Conclusion

The elements in the given list that can form molecules with one of their atom having an expanded octet are: c. P e. Se

Key concepts

Periodic Table Position and Expanded Octets

Understanding the periodic table position is crucial when determining if an element can have an expanded octet. An octet refers to the eight electrons in the valence shell that an atom typically seeks to attain stability, following the octet rule. However, certain elements have the ability to expand beyond this octet to accommodate more electrons.

The capacity to expand the octet is related to the period an element is in on the periodic table. Elements in the second period, such as Boron (B), Carbon (C), and Oxygen (O), cannot form expanded octets since their valence shell doesn't have access to the d orbitals necessary for holding extra electrons. These elements are limited to an octet as their valence shell can only fill the s and p orbitals, which accommodate a maximum of eight electrons.

On the other hand, elements from the third period, such as Phosphorus (P), and from higher periods like Selenium (Se) from the fourth period, have vacant d orbitals in their valence shell. This availability allows for more than eight electrons, hence these elements can exhibit an expanded octet. Therefore, knowing an element's position on the periodic table provides insight into its possible valence electron capacity and its likelihood to form an expanded octet.

The Role of d Orbitals in Expanded Octet Formation

The concept of d orbitals is essential when discussing expanded octets. d orbitals are a set of five orbitals, dxy, dxz, dyz, dx2-y2, and dz2, that become relevant for elements in the third period and beyond. These orbitals belong to the third shell or higher and have the capacity to hold a maximum of ten electrons, which is essential for the formation of expanded octets.

When it comes to second period elements, such as B, C, and O, the absence of accessible d orbitals restricts them to a maximum of eight valence electrons. In contrast, elements starting from the third period, including Phosphorus (P) and beyond, such as Selenium (Se), have empty d orbitals in their valence shells. This space permits the accommodation of additional electrons, enabling these elements to exceed the classical octet rule and form compounds with more than eight valence electrons.

The availability of d orbitals and their capability to be occupied during chemical bonding plays a pivotal role in the formation of molecules with expanded octets. The intrinsic electronic structure of these elements provides the foundation for complex bonding scenarios and diverse molecular geometries.

Molecular Geometry and Expanded Octets

Molecular geometry, the three-dimensional arrangement of atoms in a molecule, is deeply influenced by an atom's ability to form an expanded octet. Such expansion has significant implications for the shape and bonding patterns of molecules involving elements capable of this feat.

When an atom, like Phosphorus (P) or Selenium (Se), hosts more than eight electrons in its valence shell, the additional electrons are placed in the d orbitals. This results in unique geometries that differ from the typical shapes predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory for eight or fewer electrons. For instance, PCl5, with Phosphorus at the center, forms a trigonal bipyramidal structure, while SF6, involving Sulfur, forms an octahedral structure.

These expanded geometries are a direct result of the extra pairs of electrons occupying the d orbitals, which modify the electron-pair repulsion dynamics, thereby altering the angles between bonds and the overall spatial configuration of the molecule. Understanding how expanded octets contribute to these geometries offers a clearer insight into the complexity and versatility of molecular structures.